Ring laser gyroscopes of conventional design are limited in sensitivity at low rotation rates by the phenomenon of frequency locking. Frequency locking in ring laser gyroscopes results from the coupling of counterpropagating optical waves caused by backscatter from optical elements in the laser cavity. A variety of approaches, such as reducing the amount of light scattered from optical elements or mechanically dithering the cavity, have been used to diminish the effect of frequency locking. These techniques, however, have not been entirely satisfactory. Thus, measuring angular rates comparable to those of the Earth's rotational rate with an all-optical system remains a problem.
The subject matter of the present invention is related to that of U.S. Pat. application Ser. No. 07/582,738 entitled "Dual Optical Amplifier Ring Laser Gyroscope with Reduced Phase Locking," filed on Sept. 14, 1990, by Gerald L. Vick and assigned to the same assignee as the present application. The teachings of the Vick application are incorporated herein by reference. Vick uses a pair of semiconductor optical amplifiers (SOAs) spaced at a distance of 1/4 the length of the optical ring. The amplifiers are driven for one-half the light pulse round trip time. During the on time, each amplifier amplifies both a clockwise and a counterclockwise pulse, one after the other. This modulation scheme prevents counterpropagating light pulses from meeting in the amplifiers and eliminates coupling caused by backscatter from the amplifiers. However, the modulation scheme described by Vick does not address the problem of backscattering from optical elements other than the amplifiers, such as optical fibers or output couplers, which may also lead to frequency locking. Thus, there is a need for an improved ring laser gyroscope and an optical pulse modulation scheme to prevent frequency locking caused by residual backscatter.